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  • Journal article
    Chakravorty D, Chaudhuri B, Hui S, 2018,

    Estimation of aggregate reserve with point-of-load voltage control

    , IEEE Transactions on Smart Grid, Vol: 9, Pages: 4649-4658, ISSN: 1949-3053

    Voltage dependent loads can collectively provide a certain amount of power reserve (by virtue of the ability to change their power consumption within the stipulated voltage tolerance) which could be exploited for grid frequency regulation through voltage control at the substation/feeder or at the point ofload. The amount of such power reserve would vary with time of the day depending on the incidence of different types of voltage dependent loads and also the voltage profile across the feeders. It is important for the grid operators to know the aggregate power reserve from the voltage dependent loads during different times of the day in order to schedule other forms of reserves accordingly. This paper presents a methodology to estimate such power reserve from the measured power and voltage at the bulk supply points without knowing the actual distribution network topology and/or load profile of individual customers. The proposed method is applied to estimate the time variation of the aggregate reserve offered by the voltage dependent loads within the domestic sector in Great Britain (GB). Studies on astandard IEEE distribution network are presented to validate the estimated reserve margins under typical voltage profiles across the distribution feeders.

  • Journal article
    Yan S, Wang MH, Yang TB, Tan SC, Chaudhuri B, Hui SYRet al., 2018,

    Achieving multiple functions of 3-phase electric springs in unbalanced 3-phase power systems using the instantaneous power theory

    , IEEE Transactions on Power Electronics, Vol: 33, Pages: 5784-5795, ISSN: 0885-8993

    IEEE Three-phase electric spring (3-ph ES) has recently been proposed as a fast demand response technology for applications in unbalanced power systems fed with a mixture of conventional and renewable power generation. Using the Instantaneous Power Theory as the theoretical framework, this paper presents the criteria and conditions for minimizing the average and oscillating power of the 3-ph ES for the first time. A detailed analysis of the use of 3-ph ES is included for providing multiple control objectives of voltage regulation and power balancing of the 3-ph power system, and minimization of the average and oscillating ac power of the ES. A corresponding control scheme implementable in a single controller is included and explained. The control scheme has been practically verified with experiments.

  • Conference paper
    Chen B, Pin G, Ng WM, Hui SYR, Parisini Tet al., 2018,

    An Adaptive-Observer-Based Robust Estimator of Multi-sinusoidal Signals

    , IEEE Conferences, Publisher: IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Pages: 1618-1631, ISSN: 0018-9286
  • Journal article
    Majumdar A, Pal BC, 2018,

    Bad data detection in the context of leverage point attacks in modern power networks

    , IEEE Transactions on Smart Grid, Vol: 9, Pages: 2042-2054, ISSN: 1949-3061

    This paper demonstrates a concept to detect bad data in state estimation when the leverage measurements are tampered with gross error. The concept is based on separating leverage measurements from non-leverage measurements by a technique called diagnostic robust generalized potential (DRGP), which also takes care of the masking or swamping effect, if any. The methodology then detects the erroneous measurements from the generalized studentized residuals (GSR). The effectiveness of the method is validated with a small illustrative example, standard IEEE 14-bus and 123-bus unbalanced network models and compared with the existing methods. The method is demonstrated to be potentially very useful to detect attacks in smart power grid targeting leverage points in the system.

  • Journal article
    De Paola A, Angeli D, Strbac G, 2018,

    Integration of price-responsive appliances in the energy market through flexible demand saturation

    , IEEE Transactions on Control of Network Systems, Vol: 5, Pages: 154-166, ISSN: 2325-5870

    This paper proposes a novel decentralized technique for efficient integration of flexible demand in the electricity market. The analysis focuses on price-responsive appliances that schedule their power consumption on the basis of a demand/price signal received by a central entity. Previous work has shown that, when the devices population is sufficiently large to be described as a continuum, it is possible to provide necessary and sufficient conditions for the existence of a Nash equilibrium (no device has unilateral interest in changing its scheduling when considering the resulting profile of aggregate demand). These results are now extended in order to achieve an equilibrium also when the mentioned conditions are violated. To this purpose, a time-varying proportional constraint (equal for all devices) is introduced on the power rate of the price-responsive appliances so as to limit the variation of flexible demand that they can introduce at critical time instants. The proposed design technique not only guarantees existence of a Nash equilibrium but it also minimizes the global operation time of the appliances population. Simulation results are provided and it is shown that, under the considered assumptions, each individual appliance completes its task in minimum time.

  • Journal article
    Judge PD, Merlin MMC, Green TC, Trainer DR, Vershinin Ket al., 2018,

    Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs

    , IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 33, Pages: 349-359, ISSN: 0885-8977
  • Conference paper
    Di Franco P, Scarciotti G, Astolfi A, 2018,

    On the stability of constrained mechanical systems

    , 56th IEEE Conference on Decision and Control, Publisher: IEEE

    The problem of the stability analysis for constrained mechanical systems is addressed using tools from classical geometric control theory, such as the notion of zero dynamics. For the special case of linear constrained mechanical systems we show that stability is equivalent to a detectability property. The proposed techniques are illustrated by means of simple examples.

  • Journal article
    Gu Y, Bottrell N, Green TC, 2018,

    Reduced-order models for representing converters in power system studies

    , IEEE Transactions on Power Electronics, Vol: 33, Pages: 3644-3654
  • Journal article
    De Paola A, Angeli D, Strbac G, 2017,

    Price-Based Schemes for Distributed Coordination of Flexible Demand in the Electricity Market

    , IEEE TRANSACTIONS ON SMART GRID, Vol: 8, Pages: 3104-3116, ISSN: 1949-3053
  • Journal article
    Tzelepis D, Rousis AO, Dysko A, Booth C, Strbac Get al., 2017,

    A new fault-ride-through strategy for MTDC networks incorporating wind farms and modular multi-level converters

  • Conference paper
    Di Franco P, Scarciotti G, Astolfi A, 2017,

    A note on the stability of nonlinear differential-algebraic systems

    , 20th IFAC World Congress, Publisher: Elsevier, Pages: 7421-7426, ISSN: 1474-6670

    The problem of the stability analysis for nonlinear differential-algebraic systems is addressed using tools from classical control theory Exploiting Lyapunov Direct Method we provide linear matrix inequalities to establish stability properties of this class of systems. In addition, interpreting the differential-algebraic system as the feedback interconnection of a dynamical system and an algebraic system, a sufficient stability condition has been derived using the small-gain theorem. The proposed techniques are illustrated by means of simple examples.

  • Conference paper
    Pudjianto D, Strbac G, Boyer D, 2017,

    Virtual power plant: managing synergies and conflicts between transmission system operator and distribution system operator control objectives

    , CIRED 24th International Conference on Electricity Distribution, Publisher: IET, Pages: 2049-2052, ISSN: 2515-0855

    In this study, the implementation of virtual power plant (VPP) as a means to coordinate the use of distributed resources for different control objectives by transmission system operator and distribution system operator is described. In order to illustrate the concept, a range of illustrative studies demonstrating the application of VPP concept on a real 11 kV system in Brixton will be presented, using data from the Low Carbon London project. The studies demonstrate the changes in the operating characteristics of the VPP area over a range of system operating conditions.

  • Journal article
    Xiang X, Zhang X, Chaffey G, Green Tet al., 2017,

    An Isolated Resonant Mode Modular Converter with Flexible Modulation and Variety of Configurations for MVDC Application

    , IEEE Transactions on Power Delivery, Vol: 33, Pages: 508-519, ISSN: 0885-8977

    The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium voltage dc (MVDC) networks. The transformer-less RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step-ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a down-scaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVDC applications.

  • Journal article
    Thammawichai M, Kerrigan EC, 2017,

    Energy-efficient real-time scheduling for two-type heterogeneous multiprocessors

    , Real-Time Systems, Vol: 54, Pages: 132-165, ISSN: 0922-6443

    We propose three novel mathematical optimization formulations that solve the same two-type heterogeneous multiprocessor scheduling problem for a real-time taskset with hard constraints. Our formulations are based on a global scheduling scheme and a fluid model. The first formulation is a mixed-integer nonlinear program, since the scheduling problem is intuitively considered as an assignment problem. However, by changing the scheduling problem to first determine a task workload partition and then to find the execution order of all tasks, the computation time can be significantly reduced. Specifically, the workload partitioning problem can be formulated as a continuous nonlinear program for a system with continuous operating frequency, and as a continuous linear program for a practical system with a discrete speed level set. The latter problem can therefore be solved by an interior point method to any accuracy in polynomial time. The task ordering problem can be solved by an algorithm with a complexity that is linear in the total number of tasks. The work is evaluated against existing global energy/feasibility optimal workload allocation formulations. The results illustrate that our algorithms are both feasibility optimal and energy optimal for both implicit and constrained deadline tasksets. Specifically, our algorithm can achieve up to 40% energy saving for some simulated tasksets with constrained deadlines. The benefit of our formulation compared with existing work is that our algorithms can solve a more general class of scheduling problems due to incorporating a scheduling dynamic model in the formulations and allowing for a time-varying speed profile.

  • Journal article
    Kolesnik S, Sitbon M, Lineykin S, Batzelis E, Papathanassiou S, Suntio T, Kuperman Aet al., 2017,

    Solar Irradiation Independent Expression for Photovoltaic Generator Maximum Power Line

    , IEEE JOURNAL OF PHOTOVOLTAICS, Vol: 7, Pages: 1416-1420, ISSN: 2156-3381
  • Journal article
    Cantoni M, Farokhi F, Kerrigan EC, Shames Iet al., 2017,

    Structured computation of optimal controls for constrained cascade systems

    , International Journal of Control, ISSN: 0020-7179

    Constrained finite-horizon linear-quadratic optimal control problems are studied within the context of discrete-time dynamics that arise from the series interconnec- tion of subsystems. A structured algorithm is devised for computing the Newton-like steps of primal-dual interior-point methods for solving a particular re-formulation of the problem as a quadratic program. This algorithm has the following properties: (i) the computation cost scales linearly in the number of subsystems along the cascade; and (ii) the computations can be distributed across a linear proces- sor network, with localized problem data dependencies between the processor nodes and low communication overhead. The computation cost of the approach, which is based on a fixed permutation of the primal and dual variables, scales cubically in the time horizon of the original optimal control problem. Limitations in these terms are explored as part of a numerical example. This example involves application of the main results to model data for the cascade dynamics of an automated irrigation channel in particular.

  • Journal article
    Judge P, Chaffey G, Merlin MMC, Clemow P, Green Tet al., 2017,

    Dimensioning and modulation index selection for the hybrid modular multilevel converter

    , IEEE Transactions on Power Electronics, Vol: 33, Pages: 3837-3851, ISSN: 1941-0107

    The Hybrid MMC, comprising a mixture of fullbridgeand half-bridge sub-modules, provides tolerance to DCfaults without compromising the efficiency of the converter to alarge extent. The inclusion of full-bridges creates a new freedomover the choice of ratio of AC to DC voltage at which theconverter is operated, with resulting impact on the converter’sinternal voltage, current and energy deviation waveforms, allof which impact the design of the converter. A design methodaccounting for this, and allowing the required level of deratingof nominal sub-module voltage and up-rating of stackvoltage capability to ensure correct operation at the extremes ofthe operating envelope is presented. A mechanism is identifiedfor balancing the peak voltage that the full-bridge and halfbridgesub-modules experience over a cycle. Comparisons aremade between converters designed to block DC side faultsand converters that also add STATCOM capability. Resultsindicate that operating at a modulation index of 1.2 gives agood compromise between reduced power losses and additionalrequired sub-modules and semiconductor devices in the converter.The design method is verified against simulation results and theoperation of the converter at the proposed modulation index isdemonstrated at laboratory-scale.

  • Journal article
    Scarciotti G, Astolfi A, 2017,

    Nonlinear Model Reduction by Moment Matching

    , Foundations and Trends in Systems and Control, Vol: 4, Pages: 224-409, ISSN: 2325-6818

    Mathematical models are at the core of modern science and technology. An accurate description of behaviors, systems and processes often requires the use of complex models which are difficult to analyze and control. To facilitate analysis of and design for complex systems, model reduction theory and tools allow determining “simpler” models which preserve some of the features of the underlying complex description. A large variety of techniques, which can be distinguished depending on the features which are preserved in the reduction process, has been proposed to achieve this goal. One such a method is the moment matching approach.This monograph focuses on the problem of model reduction by moment matching for nonlinear systems. The central idea of the method is the preservation, for a prescribed class of inputs and under some technical assumptions, of the steady-state output response of the system to be reduced. We present the moment matching approach from this vantage point, covering the problems of model reduction for nonlinear systems, nonlinear time-delay systems, data-driven model reduction for nonlinear systems and model reduction for “discontinuous” input signals. Throughout the monograph linear systems, with their simple structure and strong properties, are used as a paradigm to facilitate understanding of the theory and provide foundation of the terminology and notation. The text is enriched by several numerical examples, physically motivated examples and with connections to well-established notions and tools, such as the phasor transform.

  • Journal article
    Singh AK, Pal BC, 2017,

    Decentralized nonlinear control for power systems using normal forms and detailed models

    , IEEE Transactions on Power Systems, Vol: 33, Pages: 1160-1172, ISSN: 1558-0679

    This paper proposes a decentralized method fornonlinear control of oscillatory dynamics in power systems. Themethod is applicable for ensuring both transient stability as wellas small-signal stability. The method uses an optimal control lawwhich has been derived in the general framework of nonlinearcontrol using normal forms. The model used to derive the controllaw is the detailed subtransient model of synchronous machinesas recommended by IEEE. Minimal approximations have beenmade in either the derivation or the application of the controllaw. The developed method also requires the application ofdynamic state estimation technique. As the employed control andestimation schemes only need local measurements, the methodremains completely decentralized. The method has been demon-strated as an effective tool to prevent blackouts by simulating amajor disturbance in a benchmark power system model and itssubsequent control using the proposed method.

  • Journal article
    Merlin MMC, Soto-Sanchez D, Judge PD, Chaffey G, Clemow P, Green TC, Trainer DR, Dyke KJet al., 2017,

    The extended overlap alternate arm converter: a voltage source converter with DC fault ride-through capability and a compact design

    , IEEE Transactions on Power Electronics, Vol: 33, Pages: 3898-3910, ISSN: 1941-0107

    The Alternate Arm Converter (AAC) was one ofthe first modular converter topologies to feature DC-side faultride-through capability with only a small penalty in powerefficiency. However, the simple alternation of its arm conductionperiods (with an additional short overlap period) resulted in(i) substantial 6-pulse ripples in the DC current waveform,(ii) large DC-side filter requirements, and (iii) limited operatingarea close to an energy sweet-spot. This paper presents a newmode of operation called Extended Overlap (EO) based onthe extension of the overlap period to 60◦which facilitates afundamental redefinition of the working principles of the AAC.The EO-AAC has its DC current path decoupled from the ACcurrent paths, a fact allowing (i) smooth DC current waveforms,(ii) elimination of DC filters, and (iii) restriction lifting on thefeasible operating point. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verifiedwith tests on an experimental prototype. Finally, a comparisonwith other modular converters demonstrates that the EO-AACis at least as power efficient as a hybrid MMC (i.e. a DC faultride-through capable MMC) while offering a smaller converterfootprint because of a reduced requirement for energy storagein the submodules and a reduced inductor volume.

  • Journal article
    Djapic P, Strbac G, McKenna R, Weinand J, Fichtner Wet al.,

    Assessing the implications of socioeconomic diversity for low carbon technology uptake in electrical distribution networks

    , Applied Energy
  • Conference paper
    Fatouros P, Konstantelos I, Papadaskalopoulos D, Strbac Get al., 2017,

    A stochastic dual dynamic programming approach for optimal operation of DER aggregators

    , IEEE PowerTech 2017, Publisher: IEEE

    The operation of aggregators of distributed energy resources (DER) is a highly complex task that is affected by numerous factors of uncertainty such as renewables injections, load levels and market conditions. However, traditional stochastic programming approaches neglect information around temporal dependency of the uncertain variables due to computational tractability limitations. This paper proposes a novel stochastic dual dynamic programming (SDDP) approach for the optimal operation of a DER aggregator. The traditional SDDP framework is extended to capture temporal dependency of the uncertain wind power output, through the integration of an n-order autoregressive (AR) model. This method is demonstrated to achieve a better trade-off between solution efficiency and computational time requirements compared to traditional stochastic programming approaches based on the use of scenario trees.

  • Conference paper
    Ye Y, Papadaskalopoulos, Moreira, strbacet al., 2017,

    Strategic Capacity Withholding by Energy Storage in Electricity Markets

    , 12th IEEE PES PowerTech Conference, Publisher: IEEE

    Abstract:Although previous work has demonstrated the ability of large energy storage (ES) units to exercise market power by withholding their capacity, it has adopted modeling approaches exhibiting certain limitations and has not analyzed the dependency of the extent of exercised market power on ES operating properties. In this paper, the decision making process of strategic ES is modeled through a bi-level optimization problem; the upper level determines the optimal extent of capacity withholding at different time periods, maximizing the ES profit, while the lower level represents endogenously the market clearing process. This problem is solved after converting it to a Mathematical Program with Equilibrium Constraints (MPEC) and linearizing the latter through suitable techniques. Case studies on a test market quantitatively analyze the extent of capacity withholding and its impact on ES profit and social welfare for different scenarios regarding the power and energy capacity of ES.

  • Conference paper
    Scarciotti G, Teel AR,

    Model Order Reduction for Stochastic Nonlinear Systems

    , 56th IEEE Conference on Decision and Control, Publisher: IEEE
  • Software
    Gu Y, Bottrell, Green, 2017,

    Reduced-Order Models for Representing Converters in Power System Studies

    Matlab codes of reduced-order models for representing power electronic converters in power system analyses.

  • Conference paper
    Padoan A, Astolfi A,

    Eigenvalues and Poles of a Nonlinear System: a Geometric Approach

    , 56th IEEE Conference on Decision and Control, Publisher: IEEE
  • Conference paper
    Padoan A, Astolfi,

    Model reduction by moment matching at isolated singularitiesfor linear systems: a geometric approach

    , 56th IEEE Conference on Decision and Control, Publisher: IEEE
  • Journal article
    Martinez Sanz I, Judge P, Spallarossa C, Chaudhuri B, Green Tet al., 2017,

    Dynamic overload capability of VSC HVDC interconnections for frequency support

    , IEEE Transactions on Energy Conversion, Vol: 32, Pages: 1544-1553, ISSN: 1558-0059

    In future power systems reduced overall inertiacaused by an increased dominance of asynchronous generationand interconnections would make frequency control particularlychallenging. As the number and power rating of Voltage SourceConverter (VSC) HVDC systems increases, network serviceprovision would be expected from such systems and to do sowould require overload capacity to be included in the converterspecifications. This paper studies the provision of frequencyservices from Modular Multilevel Converter (MMC)-based VSCHVDC interconnections using temperature-constrained overloadcapability. Overload of the MMC-based HVDC system is achievedthrough controlled circulating currents, at the expense of higherlosses, and subject to a control scheme which dynamicallylimits the overload available in order to keep the semiconductorjunction temperatures within operational limits. Two frequencycontrol schemes that use the obtained overload capacity toprovide frequency response during emergency conditions areinvestigated. The controllers’ performance is demonstrated inthe context of the future Great Britain transmission grid througha reduced equivalent test system. Simulation results show thateven modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primaryfrequency reserve and also reduce the loss of infeed requirementsof such interconnections.

  • Journal article
    Tindemans S, Strbac G, 2017,

    Robust estimation of risks from small samples

    , Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, Vol: 375, ISSN: 1471-2962

    Data-driven risk analysis involves the inference of probability distributions from measured or simulated data. In the case of a highly reliable system, such as the electricity grid, the amount of relevant data is often exceedingly limited, but the impact of estimation errors may be very large. This paper presents a robust non-parametric Bayesian method to infer possible underlying distributions. The method obtains rigorous error bounds even for small samples taken from ill-behaved distributions. The approach taken has a natural interpretation in terms of the intervals between ordered observations, where allocation of probability mass across intervals is well specified, but the location of that mass within each interval is unconstrained. This formulation gives rise to a straightforward computational resampling method: Bayesian interval sampling. In a comparison with common alternative approaches, it is shown to satisfy strict error bounds even for ill-behaved distributions.

  • Conference paper
    Xiang X, Zhang X, Chaffey G, Gu Y, Green Tet al., 2017,

    The isolated resonant modular multilevel converters with extreme step-ratio for MVDC application


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